This invention relates to the single-frequency, broadly tunable, and single spatial mode high power operation of semiconductor diode lasers.
The peak output power from a diode laser is proportional to the active area of the emitting facet of the device and is limited by catastrophic degradation. To increase the output power from semiconductor diode lasers, it is necessary to increase the area of the active gain region. However, monolithic diode lasers with large emitting areas produce a broad spectral output because they usually operate in many spatial modes and/or filaments. In addition, monolithic diode lasers have an output wavelength which is usually centered near the gain peak and can not be easily tuned. Large active area devices are those with a Fresnel number describing the cavity given by F=.congruent.D.sup.2 /.lambda.l&gt;1, where D is the width of the active gain region, l is the cavity length and .lambda. is the wavelength of the laser. Note that although the width of the active gain region is referred to, the height of the active gain region in most monolithic diode lasers is usually comparable to the wavelength of light and the wave is guided in this direction. The same Fresnel consideration applies to the height of the active region when the wave is not guided.
As the width of the active region increases, there is enough total gain perpendicular to the direction of the resonant cavity that stimulated emission begins to become large along this direction compared to the preferred direction. This in turn has two effects. First, it takes away energy from the preferred direction of emission and second, it contributes to the propagation of dark line defects in the device. Dark line defects are well known and are defects which can absorb both spontaneous and stimulated radiation and thereby degrade the performance of the laser. As these dark line defects increase in number and are distributed through the crystal due to optical radiation, the device is damaged further and lasing may no longer occur.
In addition, there is what is termed as filamenting. In filamenting, multiple portions of the gain region can lase independently due to such effects as nonuniform current injection into the gain region and poor uniformity of material. Instead of being uniformly radiating, the emission pattern occurs with many intense peaks. When this uncontrolled filamenting occurs, the local field intensity of laser light at the facet can exceed that which is necessary to cause catastrophic degradation.
The use of an external cavity on a semiconductor laser can overcome the problems described above. Use of an external cavity will reduce stimulated emission in all directions except that defined by the external cavity and will also reduce spontaneous emission by clamping the gain at a value lower than that usually occurring in the monolithic device thereby reducing the production rate of such dark line defects. In addition, because there is usually no spectral nor spatial hole burning in semiconductor lasers, it is possible to extract nearly all of the multimode output power from a monolithic diode laser in a single frequency when the output is controlled in a single spatial mode using an external cavity. Such external cavity devices may then be easily tuned over a broad spectral bandwidth and frequency converted using techniques described below.